In a semiconductor production process, an electrostatic chuck, is used to hold a wafer in a processing chamber of a CVD apparatus for forming a thin film on a semiconductor wafer, a dry etching apparatus for micromachining the wafer, or the like. In the fabrication of modern integrated circuit devices, one of the key requirements is the ability to construct plugs or interconnects in reduced dimensions such that they may be used in a multi-level metalization structure. The numerous processing steps involved require the formation of via holes for the plug or interconnect in a dimension of 0.5 μm or less for use in high-density logic devices. For instance, in forming tungsten plugs by a chemical vapor deposition method, via holes in such small dimensions must be formed by etching through layers of oxide and spin-on-glass materials at a high etch rate. A high-density plasma etching process utilizing a fluorine chemistry is frequently used in the via formation process.
In a modern etch chamber, an electrostatic wafer holding device, i.e., an electrostatic chuck or commonly known as an E-chuck, is frequently used where the chuck electrostatically attracts and holds a wafer that is positioned on top. The E-chuck holding method is highly desirable in the vacuum handling and processing of wafers. In contrast to a conventional method of holding wafers by mechanical clamping means where only slow movement is allowed during wafer handling, an E-chuck device can hold and move wafers with a force equivalent to several tens of Torr pressure.
Electrostatic chucking is a technique used to secure a wafer onto a susceptor in a wafer processing chamber. In more recently developed wafer processing technology, the electrostatic wafer holding technique is frequently employed in which an E-chuck electrostatically attracts and holds the wafer. It is a highly desirable technique used in the vacuum handling and processing of silicon wafers. In contrast to a conventional method of holding wafers by either gravity or mechanical clamping means only slow motion of the susceptor is allowed during wafer handling, an electrostatic wafer holding device can hold wafers with a force that is significantly higher.
Electrostatic chucks have been used to overcome the non-uniform clamping associated with mechanical clamping devices. The electrostatic chuck utilizes the attractive coulomb force between oppositely charged surfaces to clamp together an article and a chuck. It is generally recognized that in an electrostatic chuck, the force between the wafer and the chuck is uniform for a flat wafer and a flat chuck. This is in contrast to a mechanical clamping system where the clamping is effected around the peripheral of a wafer. Special provisions must be made to compensate for the bowing at the center of the wafer caused by the pressure of cooling gas which is pumped in between the wafer and the pedestal that is supporting and cooling the wafer. For instance, in order to compensate for the bowing of the wafer, one solution is to make the pedestal in a domed or bowed shape. Bowing is substantially eliminated in an electrostatic chuck where the wafer is held on a substantially planar chuck surface with an even electrostatic force distributed according to the electrode layout. The electrostatic force may prevent bowing of the wafer and thus, promote uniform heat transfer over the entire wafer surface.
In the normal operation of an electrostatic chuck, one or more electrodes formed in the chuck body induce an electrostatic charge on the surface of a dielectric material that is coated over the chuck surface facing the wafer, i.e., between the bottom surface of the wafer and the top surface of the chuck. A typical surface of the dielectric material that can be used for such purpose is, for instance, a polyamide or ceramic material. The electrostatic force between the wafer and the chuck is proportional to the square of the voltage between them and to the dielectric constant of the dielectric layer, and inversely proportional to the square of the distance between the wafer and the chuck, i.e.,Electrostatic Chucking Force=k(V/D)2 wherein k is the dielectric constant of the dielectric layer. V is the voltage drop across the dielectric film, and d is the thickness of the dielectric layer. The charging/discharging time constant is RC. When R is very large for a thick oxide backing layer (i.e., d is very large), the electrostatic chucking force can be greatly reduced causing the electrostatic chucking of the wafer to fail.
Since the principal of electrostatic chucking is that there must exist an attractive force between two parallel plates, i.e., between the silicon wafer and the susceptor that have opposite electrical charges, the chucking efficiency is not only determined by the bias voltage, the electric constant of the system, the effective distance between the two parallel plates, but also determined by the wafer grounding efficiency. To utilize electrostatic chucking efficiently in a wafer processing chamber, the surface of the wafer should be electrically conductive so that it can be properly grounded.
A typical inductively coupled plasma etch chamber 10 is shown in FIG. 1. In the etch chamber 10, which is similar to a Lam TCP® etcher made by the Lam Research Corp., the plasma source is a transformer-coupled plasma source which generates high-density, low-pressure plasma 12 decoupled from the wafer 14. The plasma source allows independent control of ion flux and ion energy. Plasma 12 is generated by a flat spiral coil 16, i.e., an inductive coil, which is separated from the plasma by a dielectric plate 18, or a dielectric window on top of the reactor chamber 20. The wafer 14 is positioned away from the coil 16 so that it is not affected by the electromagnetic field generated by the coil 16. There is very little plasma density loss because plasma 12 is generated only a few mean free paths away from the wafer surface. The Lam TCP® plasma etcher therefore enables a high-density plasma and high-etch rates to be achieved. In the plasma etcher 10, an inductive supply 22 and a bias supply 24 are used to generate the necessary plasma field. Multi-pole magnets 26 are used to hold the wafer 14 during the etching process. A wafer chuck 28 is used to hold the wafer 14 during the etching process. A ground 30 is provided to one end of the inductive coil 16.
In a typical inductively coupled RF plasma etcher 10 shown in FIG. 1, a source frequency of 13.56 MHZ and a wafer bias frequency of 13.56 MHZ are utilized. An ion density of approximately 0.5˜2×1012 cm3 at wafer, an electron temperature of 3.5 to 6 eV and a chamber pressure of 1 to 25 m Torr are achieved or used.
In the typical plasma etch chamber 10, cooling means for uniformly cooling the wafer backside is provided in an E-chuck for controlling the wafer temperature during the plasma processing. This is shown in FIG. 2 for the plasma etcher 40. Although FIG. 2 is a schematic of cooling means for a wafer backside, other cooling means may be used, such as means described in U.S. Pat. No. 5,985,035 which is herein incorporated by reference.
In the conventional plasma etcher 40, E-chuck 42 is provided for supporting a wafer 44 thereon. E-chuck 42 can be constructed of either a metallic material or a polymeric material. A plurality of ventilation apertures (not shown) are provided in the E-chuck surface such that a cooling gas can be supplied to the backside 46 of the wafer 44 during plasma processing. The plurality of ventilation apertures in the E-chuck 42 is connected in fluid communication with a cooling gas inlet conduit 38 for feeding a cooling gas into the apertures. The cooling gas inlet conduit 38 is in turn connected to a gas supply line 36, a flow control valve 34 and a cooling gas supply 32. The pressure in the cooling gas supply line 36 is monitored by a pressure sensing device 48 which is in turn sends a signal 50 to a controller 52. The controller 52, after receiving signal 50 and comparing to a pre-stored value, sends signal 54 to the flow-control valve 34 for opening or closing the valve and thus increasing or decreasing the cooling gas supply through the supply line 36, 38 into the E-chuck 42. The amount of the cooling gas that is supplied to the E-chuck 42 is further adjusted by a needle valve 56 and pumped away by a pump 58. The assembly may further include a temperature feedback control system.
FIG. 3 illustrates a cross section of a conventional electrostatic chuck assembly 60 provided to hold a wafer 62 in position for processing within an apparatus such as a processing chamber of a CVD apparatus for forming a thin film on a semiconductor wafer or a dry etching apparatus for micromachining the wafer. The assembly 60 has a wafer holding member 64 that has a vertical peripheral wall 82 coated with a dielectric material 66, and a susceptor 68 integrally formed with the wafer holding member 64. The suscepter has a side wall 70 for restricting lateral movement of the wafer 62. There is a gap 72 between a backside 74 of the wafer 62 and the susceptor 68 wherein byproducts resulting from the wafer processing may be disposed within the gap 72 and thus, move the wafer 62 so that the wafer backside 74 is not substantially planar with the wafer holding member 64. A problem with existing ESC assemblies is that byproducts of the wafer etching process can flow to the backside of the wafer and thus cause cooling gas to be unevenly distributed to the wafer 62 backside and then cause deformation of the wafer 62. Over-time, the susceptor may wear due to contact with the plasma and the susceptor is difficult to clean.
FIG. 4-6 show another conventional electrostatic chuck assembly 60. FIG. 4 illustrates a cross section of the electrostatic chuck assembly 60 that has a non-tapered insert ring 76 juxtaposed between the susceptor 68 and the wafer holding member 64. The insert ring 76 has a vertical inner wall 78 and a vertical outer side wall 80 parallel to the susceptor side wall 70. However, as shown in FIG. 5, a gap 84 is often formed between the vertical inner wall 78 of the insert ring 76 and the vertical peripheral wall 82 (also shown in FIGS. 4 and 6) of the wafer holding member 64 when the insert ring 76 moves in a non-uniform manner relative to the wafer holding member 64. Thus, the insert ring 76 is not evenly distributed around the peripheral wall 82 of the wafer holding member 64 and a processing byproduct 75, as shown in FIG. 6, could still get between the backside 74 of the wafer and the wafer holding member 62 causing cooling gas to unevenly be distributed on the wafer backside 74 and thus deform the wafer. FIG. 5 illustrates a top view of the assembly 60 having the insert ring 76 and unwanted gap 84. FIG. 6 is an enlarged view of the assembly 60 showing the gap 84 and unwanted byproduct 75.
The leakage rate of plasma in the electrostatic chuck assembly 60 shown in FIGS. 4-5 can be 4 sccm and can seriously deteriorate the insert ring 76 after having a high frequency bias voltage flow through the ESC assembly 60 over time.
Therefore, it is desirable to provide an insert ring that acts as a protective cover for the peripheral wall of a wafer holding member that can be easily removed and replaced with another insert ring to keep the ESC assembly free from foreign substances to allow for easier wet cleaning of the ESC assembly.
It is desirable to have a leakproof seal formed between a wafer holding member and a removable insert ring to prevent byproduct from forming within an ESC assembly.
It is desirable to provide an improved ESC that prevents plasma leakage to the underside of a wafer.
It is desirable to provide an improved ESC that prevents uneven distribution of helium to a wafer backside after a dry etch process of PAD and VIA has run for several hours.
It is desirable to provide an improved ESC that will improve the uniformity of a PAD layer of the wafer and provide for a uniformity in the PAD layer of less than 3%.
It is desirable to provide an improved ESC assembly that reduces the preventative maintenance time required to clean an ESC assembly.
It is further desirable to provide a method of holding a wafer and a wafer holding system in which a cooling gas can be quickly dispersed over the back surface of a wafer when the cooling gas is introduced, after the wafer is electrostatically attracted, and wafer temperature control suitable for high productivity can be performed.
It is further desirable to improve the product yield of wafer etching and the availability of the wafer etching apparatus providing a wafer holding system which is subjected to a reduced amount of foreign substances as described above and is capable of performing uniform etching.